1,2-Diarylethylenes singlet states

While the photocyclization of most cis-diarylethylenes takes place from the I st excited singlet state, there are two related systems which probably undergo cyclization from the first triplet state. 

This chapter deals with the properties of the excited states of 1,2-diary lethylenes in general, with the effects caused by specific variation of reaction conditions on photochemical cis trans isomerization in condensed phase, and with the reaction mechanisms. In particular, the influence of substitution on the properties of excited states involved in cis trans isomerization in solution is examined. Besides various substituted stilbenes (which have been most extensively studied), styrylpyridines (StPs, azastilbenes) including some of their positively charged derivatives (quaternary stilbazolium salts), dipyridylethylenes (DPEs), styrylnaphthalenes (StNs), their pyridine analogues (NPEs), and some related compounds, such as dinaphthylethylenes (DNEs), are surveyed. Results on photochemical cis ttrans isomerization of stilbenes and other 1,2-diarylethylenes under direct (Section II) and sensitized (Section III) irradiation conditions are summarized, as well as their photophysical excited singlet and triplet state properties (Section IV) and some selected side reactions (Section V). The mechanistic section (Section VI) describes several photochemical isomerization routes. Characteristic photophysical and photochemical aspects of specific classes of substituted stilbenes are discussed and mechanistic schemes are critically examined with reference to their experimental basis. 

Effects of Quenchers. The cis trans photoisomerization of 1,2-diarylethylenes can be influenced by properties of the medium, nature of the sensitizer (Section III), and also several additives acting as quenchers [192-202], The effect of quenchers on the quantum yields and the photo-stationary trans/cis ratio has been used as a mechanistic probe for exploration of isomerization pathways. Since quenching can occur with singlet as well as with triplet states, measurements on various systems have been performed under direct and sensitized excitation conditions. 

Although detailed mechanistic studies are rare, the results for several 1,2-diarylethylenes in solution at room temperature are also in agreement with a singlet mechanism. A singlet pathway may be proposed especially for a variety of substituted stilbenes (for exceptions see below), for azastilbenes [29, 274, 281] (despite the presence of n, n states) and for the two StNs... 

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